1. Field of the Invention
The present invention relates to a light-receiving element for receiving a reflection of laser light irradiated to a rotating multi-layer recording medium having a plurality of information recording layers stacked one over another and converting the reflected light into an electrical signal, an optical head having the element and recording information in a multi-layer recording medium or reproducing information recorded therein, an optical recording/reproducing apparatus, and a method of optical recording and reproduction.
2. Description of the Related Art
An optical recording/reproducing apparatus has an optical head which is formed along the circumferential direction of, for example, a disk-shaped optical recording medium (optical disk) and which records information in predetermined regions of a plurality of tracks formed in the radial direction of the optical recording medium or reproduces information recorded in predetermined regions of the tracks. Optical heads include recording-only types which are used only for recording information in an optical recording medium, reproduction-only types which are used only for reproducing information, and recording/reproduction types which can be used for both recording and reproduction. Therefore, apparatus loaded with those types of heads respectively constitute optical recording apparatus, optical reproducing apparatus, and optical recording/reproducing apparatus. In this specification, the term “optical recording/reproducing apparatus” will be used as a general term that implies all of those apparatus.
Patent Documents 1 and 2 disclose optical heads and optical disk apparatus for performing reproduction from a multi-layer optical disk having a plurality of recording layers for recording information. In the optical heads and optical disk apparatus, light reflected from a multi-layer optical disk (return light) is split into two or more optical paths which are converged on separate light-receiving sections. The light-receiving section for a first beam of light receives the entire light beam, and the light-receiving section for a second beam of light receives a central part or peripheral part of the light beam. Electrical signals obtained by photoelectrical conversion at the first and second light-receiving sections, respectively, are subjected to a differential operation at a differential amplification circuit to reproduce a reproduction signal (RF signal). A method is also disclosed, in which the second light beam is used as a focus error signal. The method for reducing inter-layer crosstalks in a multi-layer optical disk using a differential amplification circuit is advantageous in that it serves the purpose of reducing the size of an optical head and an optical disk apparatus because the method involves no focal point in a return path optical system except a light-receiving section and therefore allows an optical system to have a short optical path unlike the method disclosed in Patent Document 3 for reducing inter-layer crosstalks by using a confocal optical head.
[Patent Document 1] JP-A-1999-16200
[Patent Document 2] JP-A-2002-319177
[Patent Document 3] Japanese Patent No. 2624255
According to the method disclosed in Patent Document 1, however, one of two light beams split to perform a differential operation must be received by a light-receiving element disposed in a position where a reflected light beam from a recording layer to be reproduced (reproduced layer) converges, and the other light beam must be received by a light-receiving element disposed in a position where an inner circumferential part of a reflected light beam from a recording layer adjacent to the reproduced layer converges. Since it is not possible to dispose both of the elements in one optical path, a problem arises in that optical elements are required redundantly. According to the method disclosed in Patent Document 2, a light beam is split by two light-receiving elements into inner and outer circumferential parts, and a resultant differential signal is output as an RF signal. The method serves the purpose of providing a compact optical head because a light beam can be split by two light-receiving elements located close to each other. However, no sufficient study has been made on the size of the light-receiving elements and the configuration of the light-receiving elements to perform the splitting. There is a limitation on electrical signals to be subjected to a differential operation in that they must originate only in reflected beams from particular layers (adjoining layers), and no study has been made on how to reduce inter-layer crosstalks in a multi-layer optical disk having three or more recording layers effectively.
In a multi-layer optical disk having three or more layers, reflected light (return light) from a layer adjacent to a layer under reproduction does not necessarily constitute the inter-layer crosstalk having the most significant influence on the reproduction of an RF signal. FIG. 9 is an illustration for explaining inter-layer crosstalks which can generate noise signals in a multi-layer optical disk. Referring to FIG. 9, let us assume that a recording layer L4 is a reproduced layer from which recorded information is to be reproduced; recording layers L0 and L3 are recorded regions; and a recording layer L2 is an unrecorded region. In general, the quantity of return light attributable to reflection at an unrecorded region is greater than the quantity of return light from a recorded region.
When light irradiating the multi-layer optical disk is focused on the recording layer L4 to reproduce the information recorded on the recording layer L4, reflected light from the recording layer L3 is focused near the recording layer L2. Since the recording layer L3 and the recording layer L2 are a recorded region and an unrecorded region, respectively, the resultant inter-layer crosstalk has a relatively small influence on the signal reproduced from the recording layer L4. Reflected light from the recording layer L2 is focused near the recording layer L0, and the recording layer L0 is a recorded region. Therefore, the resultant inter-layer crosstalk has a more significant influence on the signal reproduced from the recording layer L4. Therefore, the reflected light from the recording layer L2 that is further from the recording layer L4 constitutes a more significant inter-layer crosstalk than the reflected light from the recording layer L3 adjacent to the recording layer L4, the inter-layer crosstalk having the most significant influence on an RF signal from the recording layer L4 to be reproduced and constituting a source of noises. Thus, the recording layer that generates the reflected light by which inter-layer crosstalk occurs acting as the most dominant noise source varies depending on whether each of the recording layers in the optical path is recorded or not. In this case, the return light includes noise components from other recording layers to be eliminated as reflected light components which are defocused from the position in focus by an amount that is twice an optical distance between the layers. No consideration on this point is seen in Patent Document 2.